As a method for detecting faults in an electrical apparatus such as a transformer at an early stage, there has been conventionally known a method for measuring gas components dissolved in an insulating oil (also referred to as “in-oil gas”) inside an electrical apparatus. According to this method, it is possible to determine as to whether or not there are any faults in a transformer by measuring typical decomposition gas components (in-oil gas) that are generated when the transformer is in a fault condition, for example, such as local overheating or discharge, and dissolved in an insulating oil, thereby preventing a serious accident. Therefore, this method has been widely utilized for prevention of accidents and maintenance of transformers.
Generally, an analysis of gas components dissolved in an insulating oil in a transformer is carried out in such a manner that a plurality of dissolved gas components (in-oil gas) are extracted to a gas phase part, and the extracted gas components are detected and the concentrations thereof are measured using a gas chromatograph, a gas sensor, or the like (See Patent Documents 1 and 2, for example).
Examples of a method for extracting an in-oil gas to a gas phase part include a replacement extraction method in which an inert gas is injected into an insulating oil as a sample (a sample oil), and an in-oil gas is replaced with the inert gas to thereby extract the in-oil gas, a vacuum extraction method in which a vacuum gas phase part is provided above an insulating oil which is put in an extraction container, and an in-oil gas is extracted to the gas phase part, and an equilibrium extraction method in which a sample oil is encapsulated in a vial container and the like so as to have a gas phase part therein and the vial container is shaken to thereby extract an in-oil gas to the gas phase part by gas-liquid equilibrium.
The replacement extraction method includes a stripping method in which a carrier gas of a gas chromatograph is injected into a sample oil and an extracted in-oil gas is directly introduced into the gas chromatograph, a bubbling method in which a sample oil is bubbled with an inert gas and an extracted in-oil gas is collected in a gas reservoir, and the like.
In the stripping method, an in-oil gas is extracted by using an extractor as shown in FIG. 1, for example. A brief description will be made based on FIG. 1. An insulating oil sample which has been taken in a syringe 1 is injected into an extractor 4 through a sample inlet 3. A carrier gas is then injected through a gas passage 5 into the insulating oil sample 2 which has been injected into the extractor 4 to thereby extract an in-oil gas. The gas which has been extracted (the extracted gas) flows into a gas chromatograph 9 together with the carrier gas through a branch pipe 6 which is provided in an upper part of the extractor 4, a joint 7 and an oil trap 8, and a measurement and an analysis of the extracted gas are then performed in the chromatograph 9.
By the way, when an in-oil gas analysis is carried out using the above-described various methods which have been conventionally performed, there has been widely used a method in which, in order to obtain the in-oil gas concentration in an insulating oil which is an object to be measured (which can be referred to as “a real sample”) from a result of a measurement measured by using a gas chromatograph and the like, the in-oil gas concentration in the sample is obtained by the comparison between measurement data of a gas in oil standards in which a known target gas is dissolved and measurement data of the real sample. That is, at first, a target gas the volume of which has been accurately measured is dissolved in an unused insulating oil the mass and the volume of which have been accurately measured after removing a dissolved gas therefrom to thereby prepare a gas in oil standards whose in-oil gas concentration is known. Then, an extraction is performed on the gas in oil standards in the same condition as in a measurement of the real sample. An extraction rate of an extractor to be used for the measurement is calculated from the extracted-gas concentration. Then, when an in-oil gas concentration in the real sample is calculated from a measurement result of the real sample, the in-oil gas concentration in the real sample is corrected with the extraction rate which has been obtained in advance by using the gas in oil standards to thereby calculate the in-oil gas concentration in the real sample.
However, there have been problems in that the preparation of a gas in oil standards as described above has low working efficiency, needs skilled steps, and the like. In order to solve such problems, in Patent Document 3, there has been proposed an apparatus in which the efficiency in agitating an injected standard gas is improved by using a special syringe which is provided with a gas reservoir for the standard gas and a dissolution rate of the standard gas into an insulating oil is therefore increased, thereby improving the working efficiency at the time of preparing a gas in oil standards, for example.
Although the use of the apparatus described in Patent Document 3 could lead to some improvement in the above-described problems, it is still necessary to prepare a gas in oil standards. Therefore, cumbersome and skilled steps are still needed. Further, there is a problem in that an error caused by handling at the time of preparing a gas in oil standards is unavoidable. In addition, there is also another problem in that it is extremely difficult to verify the accuracy of the prepared gas in oil standards.
In particular, when an insulating oil with high viscosity (high-viscosity oil) is used, it is difficult to transfer a sample oil, from which a dissolved gas has been removed, into a syringe. In addition, agitation is difficult to be performed when the dissolved gas is removed from the sample oil and when a target gas is dissolved in the sample oil. Therefore, it is difficult to prepare a gas in oil standards. Further, the conventional extraction method has a low extraction rate and insufficient accuracy. Accordingly; a method of analysis has not yet been established under the present circumstances.